I suspect it is plaster which would allow for reflections of mid & high frequencies.
Maybe try seating under the balcony next time a good show comes through?
I was on the front row of the balcony when I saw Genesis live in 1977 (the tour of their "Seconds Out" album). It was the first time (and one of very few) where I experienced the live performance sounded better than the studio recorded album! Obviously, that can't happen unless the band puts on a good acoustic show (and Genesis did), but the acoustics of the venue have to be on the mark as well.
Also saw John Klemmer there (maybe 1978?). It didn't "shock" me with how good it sounded like the Genesis concert did, but I didn't notice any bad effects.
The next time I see a show down there, I'll give specific attention to ceiling bounce, but I would think that the first row of the balcony would be the worst place for that! If you are sitting down at floor level, it seems like the attenuation from the inverse square of the distance would minimize the SPL of ceiling reflections. Any chance that you were seated close enough to get a reflection from the ceiling over the stage?
I have heard comments that the Fox is not acoustically the best theater for plays, but it has a pretty good reputation for concerts. The best venues should be able to do both well.
I guess a lot of Patti LuPone's work leans toward musical/opera, so maybe that relates to our opposing experiences. She may have brought out the acoustic problems that people have experienced with plays.